Hybrid arq retransmission with reordering scheme employing multiple redundancy versions and receiver/transmitter therefor

ABSTRACT

A hybrid ARQ retransmission method in a communication system, wherein data packets consisting of symbols encoded with a forward error correction (FEC) technique prior to transmission are retransmitted based on an automatic repeat request and subsequently combined with previously received data packets the symbols of said data packets being modulated by a mapping unit employing a predetermined signal constellation. The retransmitted data packets being retransmitted in form of a selected one of a plurality of different redundancy versions. According to the invention the transmitted bits of the modulated symbols are reordered over the retransmissions in accordance with the selected redundancy version.

This application is a continuation of application Ser. No. 11/484,776filed Jul. 12, 2006 (pending), which is a continuation of applicationSer. No. 10/853,266 filed May 26, 2004 (U.S. Pat. No. 7,110,470) whichis a continuation of application Ser. No. 10/298,207 filed Nov. 18, 2002(U.S. Pat. No. 6,798,846), which claims the priority of EP 01127244.0filed Nov. 16, 2001.

FIELD OF THE INVENTION

The present invention relates to a hybrid ARQ retransmission method in acommunication system. Further, the invention concerns a respectivereceiver and a transmitter.

BACKGROUND OF THE INVENTION

A common technique in communication systems with unreliable andtime-varying channel conditions is to correct errors based on automaticrepeat request (ARQ) schemes together with a forward error correction(FEC) technique called hybrid ARQ (HARQ). If an error is detected by acommonly used cyclic redundancy check (CRC), the receiver of thecommunication system requests the transmitter to send additionalinformation (data packets retransmission) to improve the probability ofcorrectly decoding the erroneous packet.

A packet will be encoded with the FEC before transmission. Depending onthe content of the retransmission and the way the bits are combined withpreviously transmitted information, S. Kaliel, Analysis of a type IIhybrid ARQ scheme with code combining, IEEE Transactions onCommunications, Vol. 38, No. 8, August 1990 and S. Kallel, R. Link, S.Bakhtiyari, Throughput performance of Memory ARQ schemes, IEEETransactions on Vehicular Technology, Vol. 48, No. 3, May 1999 definethree different types of ARQ schemes:

-   -   Type I: The erroneous received packets are discarded and a new        copy of the same packet is retransmitted and decoded separately.        There is no combining of earlier and later received versions of        that packet.    -   a Type II: The erroneous received packets are not discarded, but        are combined with additional retransmissions for subsequent        decoding, Retransmitted packets sometimes have higher coding        rates (coding gain) and are combined at the receiver with the        stored soft-information from previous transmissions,    -   Type III: the same as Type II with the constraint each        retransmitted packet is now self-decodabie. This implies that        the transmitted packet is decodable without the combination with        previous packets. This is useful if some packets are damaged in        such a way that almost no information is reusable, If all        transmissions carry identified data, this can be seen as a        special case called HARQ Type III with a single redundancy        version.

HARQ Type II and III schemes are obviously more intelligent and show aperformance gain with respect to Type I, because they provide theability to reuse information from of previously received erroneouspackets. There exist basically three schemes of reusing the redundancyof previously transmitted packets:

-   -   Soft-Combining    -   Code-Combining    -   N Combination of Soft- and Code-Combining

Soft-Combining

Employing soft-combining the retransmission packets carry identicalinformation compared with the previously received information. In thiscase the multiple received packets are combined either by asymbol-by-symbol or by a bit-by-bit basis as for example disclosed in D.Chase, Code combining: A maximum-likelihood decoding approach forcombining an arbitrary number of noisy packets, IEEE Trans. Commun.,Vol. COM-33, pp. 385-393, May 1985 or B. A. Harvey and S. Wicker, PacketCombining Systems based on the Viterbi Decoder, IEEE Transactions onCommunications, Vol. 42, No 23/4, April 1994.

In case of employing symbol-level combining, the retransmitted packetshave to carry identical modulation symbols to the previously transmittederroneous packets. In this case the multiple received packets arecombined at modulation symbol level. A common technique is the maximumratio combining (MRC), also called average diversity combining (ADC), ofthe multiple received symbols, where after N transmissions thesum/average of the matching symbols is buffered.

In case of employing bit-level combining the retransmitted packets haveto carry identical bits to the previously transmitted erroneous packets.Here, the multiple received packets are combined at bit level afterdemodulation. The bits can be either mapped in the same way onto themodulation symbols as in previous transmissions of the same packet orcan be mapped differently. In case the mapping is the same as inprevious transmissions also symbol-level combining can be applied. Acommon combining technique is the addition of calculated log-likelihoodratios (LLRs), especially if using so-called Turbo Codes for the FEC asknown for example from C. Berrou, A. Glavieux, and P. Thitimajshima,Near Shannon Limit Error-Correcting Coding and Decoding: Turbo-Codes,Proc. ICC '93, Geneva, Switzerland, pp. 1064-1070, May 1993; S. Le Goff,A. Glavieux, C. Berrou, Turbo-Codes and High Spectral EfficiencyModulation, IEEE SUPERCOMM/ICC '94, Vol. 2, pp. 645-649, 1994; and A.Burr, Modulation and Coding for Wireless Communications, PearsonEducation, Prentice Hall, ISBN 0-201-39857-5, 2001. Here, after Ntransmissions the sum of the LLRs of the matching bits is buffered.

Code-Combining

Code-combining concatenates the received packets in order to generate anew code word (decreasing code rate with increasing number oftransmission). Hence, the decoder has to be aware of how to combine thetransmissions at each retransmission instant in order to perform acorrect decoding (code rate depends on retransmissions). Code-combiningoffers a higher flexibility with respect to soft-combining, since thelength of the retransmitted packets can be altered to adapt to channelconditions. However, this requires more signaling data to be transmittedwith respect to soft-combining.

Combination of Soft- and Code-Combining

In case the retransmitted packets carry some symbols/bits identical topreviously transmitted symbols/bits and some code-symbols/bits differentfrom these ones, the identical code-symbols/bits are combined usingsoft-combing as described in the section titled “Soft-Combining” whilethe remaining code-symbols/bits will be combined using code-combining.Here, the signaling requirements will be similar to code-combining.

It has been shown in M. P. Schmitt, Hybrid ARQ Scheme employing TCM andPacket Combining, Electronics Letters Vol. 34, No. 18, September 1998that HARQ performance for Trellis Coded Modulation (TCM) can be enhancedby rearranging the symbol constellation for the retransmissions. There,the performance gain results from the maximizing the Euclidean distancesbetween the mapped symbols over the retransmissions, because therearrangement has been performed on a symbol basis. Consideringhigh-order modulation schemes (with modulation symbols carrying morethan two bits) the combining methods employing soft-combining have amajor drawback: The bit reliabilities within soft-combined symbols willbe in a constant ratio over all retransmissions, i.e. bits which havebeen less reliable from previous received transmissions will still beless reliable after having received further transmissions and,analogous, bits which have been more reliable from previous receivedtransmissions will still be more reliable after having received furthertransmissions. Generally, HARQ schemes do not take into account thevariations in bit-reliabilities. These variations downgrade the decoderperformance significantly. Mainly, the variations result from tworeasons.

First, the varying bit reliabilities evolve from the constraint oftwo-dimensional signal constellation mapping, where modulation schemescarrying more than 2 bits per symbol cannot have the same meanreliabilities for all bits under the assumption that all symbols aretransmitted equally likely, The term mean reliabilities is consequentlymeant as the reliability of a particular bit over all symbols of asignal constellation.

Employing a signal constellation for a 16 QAM modulation schemeaccording to FIG. 1 showing a Gray encoded signal constellation with agiven bit-mapping order i₁q₁i₂q₂, the bits mapped onto the symbolsdiffer significantly from each other in mean reliability in the firsttransmission of the packet. In more detail, bits i₁ and q₁ have a highmean reliability, as these bits are mapped to half spaces of the signalconstellation diagram with the consequences that their reliability isindependent from the fact of whether the bit transmits a one or a zero.

In contrast thereto, bits i₂ and q₂ have a low mean reliability, astheir reliability depends on the fact of whether they transmit a one ora zero. For example, for bit i₂, ones are mapped to outer columns,whereas zeros are mapped to inner columns. Similarly, for bit q₂, onesare mapped to outer rows, whereas zeros are mapped to inner rows.

For the second and each further retransmissions the bit reliabilitieswill stay in a constant ratio to each other, which is defined by thesignal constellation employed in the first transmission, i.e. bits i₁and q₁ will always have a higher mean reliability than bits i₂ and q₂after any number of retransmissions.

Second, employing partly soft-combining, suppose that all transmittedbits would have identical reliability after the first transmission. Eventhen variations in bit reliabilities would be introduced overretransmissions, because reliabilities for those bits which areretransmitted (and soft-combined) would increase, whereas reliabilitiesof not retransmitted bits would stay unchanged. Moreover, bits which arenot transmitted in the first transmission and then transmitted inretransmissions (transmitting additional redundancy) emphasize thiseffect.

In co-pending PCT/EP01/01982 a method has been suggested that in orderto enhance the decoder performance, it would be quite beneficial to haveequal or near to equal mean bit reliabilities after each receivedtransmission of a packet. Hence, the bit reliabilities are tailored overthe retransmissions in a way that the mean bit reliabilities getaveraged out. This is achieved by choosing a predetermined first and atleast second signal constellation for the transmissions, such that thecombined mean bit reliabilities for the respective bits of alltransmissions are nearly equal. I. e. bits which have been highlyreliable in the first transmission are mapped in such a way that theybecome less reliable in the second transmission and vice versa.

Hence, the signal constellation rearrangement results in a changed bitmapping, wherein the Euclidean distances between the modulation symbolscan be altered from retransmission to retransmission due to the movementof the constellation points. As a result, the mean bit reliabilities canbe manipulated in a desired manner and averaged out to increase theperformance the FEC decoder at the receiver.

In the solution proposed above, the benefits of the constellationrearrangement are realized for the concept of the HARQ TYPE II/IIIsingle redundancy version schemes.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a hybrid ARQretransmission method and transmitter, which effectively avoidsdowngrading of the decoder performance caused by the variations in bitreliabilities.

The object is solved by a method, transmitter and receiver as set forthin the independent claims.

The invention is based on the recognition that the conventional schemesdo not consider this specific content (set of bits) of each transmissionfor reordering the bits. Hence, in order to obtain a performance gain,the reordering has to be done depending on the content of eachtransmitted redundancy version. Consequently, the invention can be seenas providing a HARQ Type-II/III scheme using multiple redundancyversions under consideration of the content of the transmittedredundancy version. This results in a significant gain in the decoderperformance.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, preferred embodiments whichwill be described in the following with reference to the accompanyingdrawings show:

FIG. 1: an exemplary constellation illustrating a 16 QAM modulationscheme with Gray-encoded bit symbols,

FIG. 2: two examples for signal constellations for a 16 QAM modulationscheme with Gray-encoded bit symbols,

FIG. 3: a generated bit sequence from a rate 1/3 FEC encoder,

FIG. 4: a chosen sequence for a rate 1/2 transmission system generatedfrom the sequence shown in FIG. 3 with an indication of the bitreliabilities,

FIG. 5: a bit sequence for the second transmission, wherein the bits areshifted by two to the right,

FIG. 6: a bit sequence for the second transmission, wherein the bitpositions are switched using different mappers.

FIG. 7: a bit sequence for the first transmission redundancy version 1and a first pair of mapper/interleaver,

FIG. 8: a bit sequence for the second transmission for a redundancyversion 2 with the same mapper/interleaver as for the firsttransmission,

FIG. 9: a bit sequence for the second transmission a redundancy version2 with different mappers/interleavers as for the first transmission,

FIG. 10: resulting bit sequences from possible combinations ofredundancy versions and mappers/interleavers,

FIG. 11: a first embodiment of a communication system in which themethod of the present invention is carried out,

FIG. 12: a second embodiment of a communication system in which themethod of the present invention is carried out,

FIG. 13: a diagram indicating the performance of several conventionalstrategies versus the strategy according to the method of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following the concept of a Log-Likelihood-Ratio (LLR) will bedescribed as a metric for the bit reliabilities. First the straightforward calculation of the bit LLRs within the mapped symbols for asingle transmission will be shown. Then the LLR calculation will beextended to the multiple transmission case.

Single Transmission

The mean LLR of the i-th bit b_(n) ^(i) under the constraint that symbols_(n) has been transmitted for a transmission over a channel withadditive white gaussian noise (AWGN) and equally likely symbols yields

$\begin{matrix}{{{{LLR}_{b_{n}^{i}{r_{n}}}\left( r_{n} \right)} = {{\log \left\lbrack {\sum\limits_{({m{{b_{m}^{i} = b_{n}^{i}})}}}\; ^{{- \frac{E_{S}}{N_{0}}} \cdot d_{n,m}^{2}}} \right\rbrack} - {\log \left\lbrack {\sum\limits_{({m{{b_{m}^{i} \neq b_{n}^{i}})}}}^{{- \frac{E_{S}}{N_{0}}} \cdot d_{n,m}^{2}}} \right\rbrack}}},} & (1)\end{matrix}$

where r_(n)=s_(n) denotes the mean received symbol under the constraintthe symbol s_(n) has been transmitted (AWGN case), d_(n,m) ² denotes thesquare of the Euclidean distance between the received symbol r_(n) andthe symbol s_(m), and E_(S)/N₀ denotes the observed signal-to-noiseratio.

It can be seen from Equation (1) that the LLR depends on thesignal-to-noise ratio E_(S)/N₀ and the Euclidean distances d_(n,m)between the signal constellation points.

Multiple Transmissions

Considering multiple transmissions the mean LLR after the k-thtransmission of the i-th bit b_(n) ^(i) under the constraint thatsymbols s_(n) _((j)) have been transmitted over independent AWGNchannels and equally likely symbols yields

$\begin{matrix}{{{{LLR}_{b_{n}^{i}{{\bigcap_{j = 1}^{k}r_{n}^{(i)}}}}\left( {r_{n}^{(1)},r_{n}^{(2)},\ldots \mspace{14mu},r_{n}^{(k)}} \right)} = {{\log \left\lbrack {\sum\limits_{({m{{b_{m}^{i} = b_{n}^{i}})}i}}\; ^{- {\sum\limits_{j = 1}^{k}\; {{(\frac{E_{S}}{N_{0}})}^{(j)}{(d_{n,m}^{(i)})}^{2}}}}} \right\rbrack} - {\log \left\lbrack {\sum\limits_{m{{b_{m}^{i} \neq b_{n}^{i}})}}\; ^{- {\sum\limits_{j = 1}^{k}\; {{(\frac{E_{S}}{N_{0}})}^{(j)} \cdot {(d_{n,m}^{(i)})}^{2}}}}} \right\rbrack}}},} & (2)\end{matrix}$

where j denotes the j-th transmission ((j−1)-th retransmission).Analogous to the single transmission case the mean LLRs depend on thesignal-to-noise ratios and the Euclidean distances at each transmissiontime.

It is clear to a skilled person, that an approximation of the LLRs canbe obtained by a simplified calculation to the above detailed equations.

In the following, the case of a 16-QAM system will be exemplarilyconsidered resulting in 2 high reliable and 2 low reliable bits, wherefor the low reliable bits the reliability depends on transmitting a oneor a zero (see FIG. 1). Hence, overall there exist 2 levels ofreliabilities wherein the second level can be further subdivided.

Level 1 (High Reliability, 2 bits): Bit mapping for ones (zeros)separated into the positive (negative) real half space for the i-bitsand the imaginary half space the q-bits. Here, there is no differencewhether the ones are mapped to the positive or to the negative halfspace.

Level 2 (Low Reliability, 2 bits): Ones (zeros) are mapped to inner(outer) columns for the i-bits or to inner (outer) rows for the q-bits.Since there is a difference for the LLR depending on the mapping to theinner (outer) columns and rows, Level 2 is further classified:

Level 2a: Mapping of i_(n) to inner columns and q_(n)to inner rowsrespectively.

Level 2b: Inverted mapping of Level 2a: Mapping of i_(n) to outercolumns and q_(n) to outer rows respectively.

To ensure an optimal averaging process over the transmissions for allbits the levels of reliabilities have to be altered.

It has to be considered that the bit-mapping order is open prior initialtransmission, but has to remain through retransmissions, e.g.bit-mapping for initial transmission: i₁q₁i₂q₂

_(bit-mapping all retransmissions: i) ₁q₁i₂q₂.

Some examples for possible constellations are shown in FIG. 2. Theresulting bit reliabilities according to FIG. 2 are given in Table 1.

TABLE 1 Constellation bit i₁ bit q₁ bit i₂ bit q₂ 1 High ReliabilityHigh Reliability Low Reliability Low Reliability (Level 1) (Level 1)(Level 2b) (Level 2b) 2 Low Reliability Low Reliability High ReliabilityHigh Reliability (Level 2a) (Level 2a) (Level 1) (Level 1) 3 LowReliability Low Reliability High Reliability High Reliability (Level 2b)(Level 2b) (Level 1) (Level 1) 4 High Reliability High Reliability LowReliability Low Reliability (Level 1) (Level 1) (Level 2a) (Level 2a)

In the following, it is assumed that m denotes the retransmission numberparameter, with m=0 denoting the first transmission of a packet in theARQ context. Further let b denote the number of bits that form a symbolin the mapping entity. Typically, b can be any integer number, where themost often used values for communication systems are an integer power of2.

Without loss of generality it can be further assumed that the number ofbits n that are used as input to the interleaving process is dividableby b, i.e. n is an integer multiple of b. Those skilled in the art willperceive that if this should not be the case, then the sequence of inputbits can be easily appended by dummy bits until the above condition ismet.

In the following an example of a simple Gray-mapped 16-QAM transmissionscheme with FEC rate 1/2 (S_(n): systematic bits—P_(n): parity bits),which is generated from a systematic encoder of rate 1/3 (see FIG. 3) bypuncturing will be considered. A sequence and ordering of bits as shownin FIG. 4 could be selected for the 1^(st) transmission (TX). FIG. 4shows the generated sequence of FIG. 3 with an indication of the bitreliabilities.

A simple conventional HARQ Type-III scheme with a single redundancyversion would transmit in all requested retransmissions the identicalsequence (having the identical mapping M₁ or identical interleaving I₁).The 1^(st) transmission is usually not interleaved, however also not tointerleave can be viewed as having an interleaver with equal input andoutput streams. This results after combining all received (andrequested) transmissions in large variations of bit reliabilities. E.g.S₁ and P₁ would be highly reliable (transmitted n times with highreliability) whereas S₂ and P₄ would be less reliable (transmitted ntimes with low reliability). As stated earlier, this will downgradedecoding performance at the receiver.

The performance of this basic scheme can be increased by switching thereliabilities for required retransmissions to average out thereliabilities for all transmitted bits. This can be achieved by a numberof different specific implementations, where 2 possible solutions aredepicted below in FIG. 5 and FIG. 6. This technique can be implementedeither by interleaving the bits differently than in the 1st transmissionor by using different mapping rules for the modulation symbols. In thefollowing this will be denoted as using a 2^(nd) mapper M₂ or a 2^(nd)interleaver I₂.

FIG. 5 shows a bit sequence for the 2^(nd) transmission, wherein, inorder to average bit reliabilities, the bits are shifted by 2 to theright using different interleavers for transmission.

FIG. 6 shows a bit sequence for the 2^(nd) transmission, wherein, inorder to average bit reliabilities, the bit positions are switched usingdifferent mappers for transmissions.

In case of using just 2 different mappers (M_(n)) or interleavers(I_(n)) all successive transmissions are then mapped or interleaved suchthat no mapper/interleaver is used 2 times more often than the otherone, e.g.:

TABLE 2 TX Strategy 1 Strategy 2 1 I₁/M₁ I₁/M₁ 2 I₂/M₂ I₂/M₂ 3 I₁/M₁I₂/M₂ 4 I₂/M₂ I₁/M₁ 5 I₁/M₁ I₁/M₁ 6 I₂/M₂ I₂/M₂ 7 I₁/M₁ I₂/M₂ . . . . .. . . .

It should be noted that for 16-QAM the usage of 4 different mappersprovides a better performance and just using 2 mappers provides a suboptimum solution. 2 mappers are chosen to keep the example simple.

It can be seen from the table above the performances of the strategy 1and 2 are equal or similar, hence, it does not make a difference ifchoosing mapper/interleaver M₁/I₁ or M₂/I₂ for the 3^(rd) TX(transmission). For the 4^(th) TX, however, it has to be taken care tochoose the complementary mapper/interleaver with respect to the 3^(rd)TX.

A simple prior art HARQ Type-III scheme with multiple redundancyversions would retransmit the systematic bits in the 2^(nd) TX plus theadditional parity bits, which have not been transmitted in the first TX.For simplicity the example is chosen such that the number of bits pertransmissions is kept constant and exactly 2 transmissions can carry allencoded bits (systematic and parity). To guarantee self-decodableretransmissions all systematic bits are retransmitted. It will howeverbe appreciated by those skilled in the art, that also non-self decodableretransmissions can be used to carry out the invention.

FIG. 7 shows a bit sequence for the 1^(st) TX as RV₁ & M₁ ¹/I₁ ¹.

For conventional schemes with multiple redundancy versions—not takingthe variations in bit reliabilities into account, i.e. having a singlemapper/interleaver as shown in the bit sequence for the sequence for the2nd transmission RV₂ & M₁ ²/I₁ ² in FIG. 8—a similar problem arises asfor schemes with a single redundancy version. Low reliable systematicbits from the 1^(st) TX will be low reliable in the 2^(nd) transmission.

Using 2 mappers/interleavers (see FIG. 9) the averaging will beperformed for the systematic bits. However, after 2 transmissionsaveraging of the reliabilities is only possible for the bits transmittedtwice so far (in this example the systematic bits). In the 3rd TX one isfree of choice which redundancy version to transmit RV₁ or RV₂(performance for both possibilities should be very similar).

The example described above having 2 redundancy versions (RV₁ and RV₂)basically provides 4 combinations of redundancy versions andmappers/interleavers (see Table 3 and FIG. 10):

TABLE 3 Possible Combinations RV₁ & I₁ ¹/M₁ ¹ RV₁ & I₂ ¹/M₂ ¹ RV₂ & I₁²/M₁ ² RV₂ & I₂ ²/M₂ ²

In the following the set of bits transmitted in the 1^(st) TX will belabeled RV₁ (redundancy version 1) and the set of bits transmitted inthe 2^(nd) TX will be labeled RV₂. Also, the mappers/interleavers arelinked to the redundancy versions by a superscript. In the shown examplethe interleaver pattern and mapping for I_(n) ¹/M_(n) ¹ and I_(n)²/M_(n) ² (n=1, 2) are equal, which is a special case, because thepositions of the systematic and parity bits are aligned to each other inboth redundancy versions.

In accordance with the present invention, the mapper/interleaver has tobe selected according to the chosen redundancy version in order toaverage out the reliabilities of the systematic and parity bits. This iscontrary to the single redundancy version case, whereby the thirdtransmission one can select any mapper/interleaver.

In the following, a strategy for selecting the mapper/interleaverdepending on the transmitted redundancy version in order to average outall bit reliabilities is proposed.

1^(st) TX

Let us assume the combinations RV₁& I₁ ¹/M₁ ¹ is selected for the 1^(st)TX—any other combination could also be selected for 1^(st) transmission(assuming equal/similar performance considering a single transmission).

2^(nd) TX

In the 2^(nd) TX the remaining redundancy version should be transmitted(in this case RV₂), where the reliabilities for all bits which have beenalready transmitted in the 1^(st) TX (in this case all systematic bits)have to be averaged, i.e. low reliable systematic bits have to be highreliable now. This is achieved by transmitting RV₂ with I₂ ²/M₂ ².

3^(rd) TX

For the 3^(rd) TX one is free which redundancy version to transmit,however it has to be combined with a mapper/interleaver, which has notbeen yet chosen for this redundancy version, i.e. RV₁ & I₂ ¹/M₂ ¹ instrategy 1 and RV₂ I₁ ²/M₁ ² in strategy 2. This ensures the averagingof the parity bits, which are transmitted in the current set of bits.

4^(th) TX

For the 4^(th) TX the combination, which is left over has to beselected. This guarantees the averaging of the remaining set of paritybits and makes sure to transmit the set of parity bits, which have justbeen transmitted once so far.

5^(th) and Further TX

After the 4^(th) TX the averaging process is finished. Hence there is afree choice of redundancy version and mapper/interleaver combination.For following TXs the rules applied to TXs 1-4 have to be considered.

TABLE 4 TX Strategy 1 Strategy 2 1 RV₁ & RV₁ & I₁ ¹/M₁ ¹ I₁ ¹/M₁ ¹ 2 RV₂& RV₂ & I₂ ²/M₂ ² I₂ ²/M₂ ² 3 RV₁ & RV₂ & I₁ ²/M₁ ² I₂ ¹/M₂ ¹ 4 RV₂ &RV₁ & I₂ ¹/M₂ ¹ I₁ ²/M₁ ² 5 . . . . . .

In the provided example the positions of the systematic bits for bothredundancy version RV₁ and RV₂ (considering same mapper/interleaver) areequal (see FIG. 10). This is not generally the case (especially fordifferent coding rates) and is clearly a simplification. The shownexample is intended to show the general procedure, which can be easilyextended to more general cases mentioned below.

The proposed method is not restricted to 2 redundancy versions. Insteadit can be extended to any number N of redundancy versions, which areselected to be transmitted consecutively and repeated after Ntransmissions as in a general HARQ Type II/III scheme with N redundancyversions.

Under the assumption that m denotes the actual mapper/interleaverversion (m=1 . . . M) the number of mappers/interleavers per redundancyversion might be any integer number M (resulting in at most into N·Mdifferent mappers/interleavers, where N denotes the total number ofredundancy versions and M the number of mappers/interleavers perredundancy version), where the mapping rules or interleaver patterns arenot necessarily designed to perform a perfect averaging ofreliabilities. According to the example in Table 4, the general methodis shown in Table 5, where (as mentioned earlier) all I_(m) ^(n)/M_(m)^(n) might have different mapping rules or interleaver patterns.

TABLE 5 TX Combination 1 RV₁ & I₁ ¹/M₁ ¹ 2 RV₂ & I₁ ²/M₁ ² 3 RV₃ & I₁³/M₁ ³ . . . . . . N RV_(N) & I₁ ^(N)/M₁ ^(N) N + 1 RV₁ & I₂ ¹/M₂ ¹ . .. . . . 2N RV_(N) & I₂ ^(N)/M₂ ^(N) . . . . . . N · (M − 1) + 1 RV₁ &I_(M) ¹/M_(M) ¹ . . . . . . N · M RV_(N) & I_(M) ^(N)/M_(M) ^(N) . . . .. .

As shown in the example, the mappers/interleavers I_(m) ^(n)/M_(m) ^(n)could be the same for all redundancy versions n, i.e.mappers/interleavers are independent from n: I_(m)/M_(m) (in total Mdifferent mappers/interleavers). The mapping rules or interleaverpatterns might be chosen such that the averaging process for both thesystematic bits and parity bits is as good as possible. Any pair ofmappers/interleavers I_(m) ^(n)/M_(m) ^(n), I_(k) ^(j)/M_(k) ^(j) mighthave the same mapping rule or interleaver pattern.

Preferably, the number M of mappers/interleavers might be chosenaccording to the number of bit-reliability levels caused by themodulation scheme. Alternatively, the number M of mappers/interleaversmight be chosen according to the twice the number of bit-reliabilitylevels caused by the modulation scheme.

FIG. 11 shows an exemplary first embodiment of a communication system inwhich the method underlying the invention is employed.

At the transmitter 100, a bit sequence is obtained from a forward errorcorrection (FEC) encoder (not shown) and subsequently input into aninterleaver 110 and a logical bit inverter 120. The interleaver 110 andlogical bit inverter 120 are each functions of the redundancy versionand/or the mapper/interleaver version m and modify the input bitsequence. Subsequently, the bit sequence is input into themapper/modulator 130 being the mapping entity. The mapper typically usesone of the signal constellations shown in FIG. 2 and maps the bits ontoa symbol which is transmitted over the communication channel 200. Thecommunication channel is typically a radio communication channelexperiencing unreliable and time-varying channel conditions.

The patterns used by the mappers, interleavers and inverters are eitherstored at both, the transmitter and the receiver or stored at thetransmitter and signalled to the receiver.

At the receiver 300, the complex symbols are first input into ade-mapper/demodulator 330 which demodulates the received symbols into acorresponding bit domain sequence (e.g. sequence of LLRs). This sequenceis then input into a logical inverter 320 and subsequently into ade-interleaver 310 from which the obtained bit domain sequence isoutput.

The interleaver and de-interleaver operate in accordance with the wellknown technique of interleaving/deinterleaving by applying a determined,pseudo-random or random permutation of the input bit or symbolsequences, i.e. change the positions of the bits or symbols within asequence, In the above described embodiment, the interleaver (and thedeinterleaver) are a intra-symbol bit (de-)interleaver which change theposition of the bits that form a symbol in the mapper/demapper.

The logical bit inverter operates in accordance with a well knowntechnique of inverting the logical value of a bit, i.e. turns a logicallow to a logical high value and vice versa. In one practical realizationof a receiver working with log likelihood ratios, this invertingoperation is equivalent to a sign inversion of the log likelihood ratio.

If a retransmission is launched by an automatic repeat request issued byan error detector (not shown) with the result that another data packetis transmitted from the transmitter 100, in the de-mapper/demodulator330, the previously received erroneous data packets are combined withthe retransmitted data packets. Due to the modification of the bitsequence by the interleaver and the logical bit inverter, the mean bitreliabilities are averaged out resulting in an increased performance inthe receiver.

As an alternative approach, in the second embodiment shown in FIG. 12,the pattern for interleaving/de-interleaving the bit sequence beforesending same to the mapper is left constant to i.e. does not change as afunction of the redundancy version n. Instead, the rules for mapping thebits onto a symbol are changed which corresponds to having input bitsequences into the mapper only depending on the redundancy version n andsimply changing the bit-to-symbol mapping rules.

In a further variant, not explicitly shown in the figures, a combinationof the two described approaches above can be used, i.e.mapper/interleaver and inverter depend on the redundancy version n andthe mapper/interleaver version m.

FIG. 13 shows the result of a simulation measuring the frame error ratefor a 16-QAM modulation scheme employing a code rate 1/2 for twoconventional HARQ methods and one possible implementation of the methodaccording the present invention. For this example, strategy 2 in belowtable 5 has been compared with two conventional strategies. It isobvious from FIG. 13 that the method according to the inventionoutperforms the conventional methods.

TABLE 5 Conventional 1 Conventional 2 (using identical (alternatingbetween Strategy 2 Scheme mapping for all mappings irrespective(according Transmission transmissions) of redundancy version) to Table3) 1. TX RV₁ & Mapping 1 RV₁ & M¹ RV₁ & M¹ (M₁) 2. TX RV₂ & M¹ RV₂ & M²RV₂ & M² 3. TX RV₁ & M¹ RV₁ & M¹ RV₁ & M² 4. TX RV₂ & M¹ RV₂ & M² RV₂ &M¹

In the table, the used redundancy versions (RV_(n)) and mappings (M^(m))for simulated methods are listed, where the mappings M₁ ¹=M₂ ¹=M¹ and M₁²=M₂ ²=M² are according to Table 4 (i.e. identical mappings used forboth redundancy versions). M¹ corresponds to constellation 1 and M²corresponds to constellation 2 in FIG. 2.

Although the method described above has been described usingGray-encoded signals and a QAM modulation scheme, it is clear to askilled person that other suitable encoding and modulation schemes, e.g.PSK-modulation can be equally used in obtaining the benefits of theinvention.

1. A transmission apparatus for transmitting data using a HARQ processcomprising a transmission section that transmits data using onecombination of a plurality of combinations, each of the combinationscomprising one redundancy version of a plurality of redundancy versionsand one constellation version of a plurality of constellation versions,at least one redundancy version of the plurality of redundancy versionsbeing assigned to more than one of the constellation versions.
 2. Thetransmission apparatus according to claim 1, wherein each of theconstellation versions defines at least one of (i) bit positions in abit sequence comprising a plurality of bits and (ii) logical values ofthe bits.
 3. The transmission apparatus according to claim 1, whereinone of the constellation versions is produced by at least one of (i)exchanging a bit with another in a bit sequence comprising a pluralityof bits and (ii) inverting a logical value of a bit in the bit sequence.4. The transmission apparatus according to claim 1, further comprising:a rearranging section that rearranges a bit sequence comprising aplurality of bits according to a constellation version of the one of thecombinations, to produce a rearranged bit sequence.
 5. The transmissionapparatus according to claim 1, further comprising: a rearrangingsection that operates to at least one of (i) exchange a bit with anotherin a bit sequence comprising a plurality of bits and (i) invert alogical value of a bit in the bit sequence.
 6. The transmissionapparatus according to claim 1, wherein said transmitting section (i)transmits first data using one of the combinations and (ii) transmitssecond data using another of the combinations.
 7. The transmissionapparatus according to claim 1, wherein said transmission section (i)transmits data using first combination of the combinations and (ii)retransmits data using second combination of the combinations.
 8. Atransmission method for transmitting data using a HARQ processcomprising a step of: transmitting data using one combination of aplurality of combinations, each of the combinations comprising oneredundancy version of a plurality of redundancy versions and oneconstellation version of a plurality of constellation versions, at leastone redundancy version of the plurality of redundancy versions beingassigned to more than one of the constellation versions.
 9. Thetransmission method according to claim 8, wherein each of theconstellation versions defines at least one of (i) bit positions in abit sequence comprising a plurality of bits and (ii) logical values ofthe bits.
 10. The transmission method according to claim 8, wherein oneof the constellation versions is produced by at least one of (i)exchanging a bit with another in a bit sequence comprising a pluralityof bits and (ii) exchanging a logical value of a bit in the bitsequence.
 11. The transmission method according to claim 8, furthercomprising: operating to at least one of (i) exchange a bit with anotherin a bit sequence comprising a plurality of bits and (ii) invert alogical value of a bit in the bit sequence.
 12. The transmission methodaccording to claim 8, wherein the transmitting step transmits data usingfirst combination of the combinations, further comprising a step of:retransmitting data using second combination of the combination.
 13. Areception apparatus for receiving data transmitted using a HARQ processcomprising: a receiving section that receives data transmitted using onecombination of a plurality of combinations, each of the combinationscomprising one redundancy version of a plurality of redundancy versionsand one constellation version of a plurality of constellation versions,at least one redundancy version of the redundancy versions beingassigned to more than one of the constellation versions.
 14. Thereception apparatus according to claim 13, further comprising: ademodulating section that demodulates the received data.
 15. Thereception apparatus according to claim 13, wherein each of theconstellation versions defines at least one of (i) bit positions in abit sequence comprising a plurality of bits and (ii) logical values ofthe bits.
 16. The reception apparatus according to claim 13, wherein oneof the constellation versions is produced by at least one of (i)exchanging a bit with another in a bit sequence comprising a pluralityof bits and (ii) inverting a logical value of a bit in the bit sequence.17. The reception apparatus according to claim 13, further comprising: arearranging section that operates to at least one of (i) exchange a bitwith another in a bit sequence comprising a plurality of bits of thereceived data and (ii) invert a logical value of a bit in the bitsequence.
 18. The reception apparatus according to claim 13, whereinsaid receiving section (i) receives data transmitted using firstcombination of the combinations and (ii) receives data retransmittedusing second combination of the combinations.
 19. A reception method forreceiving data transmitted using a HARQ process comprising a step of:receiving data transmitted using one combination of a plurality ofcombinations, each of the combinations comprising one redundancy versionof a plurality of redundancy versions and one constellation version of aplurality of constellation versions, at least one redundancy version ofthe redundancy versions being assigned to more than one of theconstellation versions.
 20. The reception method according to claim 19,further comprising: demodulating the received data.
 21. The receptionmethod according to claim 19, further comprising: operating to at leastone of (i) exchange a bit with another in a bit sequence comprising aplurality of bits of the received data and (ii) invert a logical valueof a bit in the bit sequence.
 22. The reception method according toclaim 19, wherein receiving step receives data transmitted using firstcombination of the combinations, further comprising a step of: receivingdata retransmitted using second combination of the combinations.
 23. Atransmission system comprising: a transmission apparatus comprising atransmission section that transmits data using one combination of aplurality of combinations, each of the combinations comprising oneredundancy version of a plurality of redundancy versions and oneconstellation version of a plurality of constellation versions, at leastone redundancy version of the plurality of redundancy versions beingassigned to more than one of the constellation versions, and a receptionapparatus comprising a reception section that receives data transmittedby the transmission apparatus.